1. Field of the Invention
This invention relates to a four-electrode tandem submerged arc welding process, and more particularly a four-electrode welding process capable of performing a submerged arc welding with high efficiency and high quality for manufacturing large-diameter steel pipes.
2. Description of the Prior Art
The submerged-arc welding process has been widely used for welding of large-diameter steel pipes which have been once formed in U-shaped cross-section by U-shaped dies and then in O-shaped cross-section by O-shaped dies. It has been strongly required to speed up the submerged arc welding in view of the requirement to improve the production efficiency in response to the rapidly increased demand of the large-diameter steel pipes, because the welding speed relates directly to the improvement of the production efficiency. Various investigations have been effected in order to improve the welding speed. To increase the number of electrodes is one of the solutions.
The three-electrode welding process has been mainly used for this purpose, which has brought about a great improvement of the welding speed in comparison with the two-electrode welding process. However, the welding speed achieved by the three-electrode welding process does not meet the welding speed now required.
There are many factors which would limit the welding speed, among which following two factors are important in the most cases:
(1) insurance of sufficient penetration depth, and
(2) avoidance of welding defects.
In general, the welding speed is determined by selecting the welding heat input (electric current.times.voltage/welding speed) to fulfil the condition of (1). In order to fulfil the condition at higher welding speeds, however, the electric current must be unavoidably increased. The increased current tends to increase "undercuts" which are grooves melted into the base metal adjacent to the toe of a weld which are left unfilled with weld metal and are thus opposed to the condition of (2). Moreover, if the welding speed is not increased enough to meet the increased current, the welding heat input is unduly increased to adversely affect the toughness of the welded metal. Under the circumstances, the three-electrode welding method cannot more increase the welding speed beyond that achieved at this stage.
In view of this, the four-electrode welding process is essential in order to rapidly increase the welding speed. However, the multielectrode process will cause electromagnetic interference of electrode current and ground current. Accordingly, a proper selection of current phases or connections between electrodes and power source which determine the interference is essential. If the selection is not correct, the welded quality becomes inferior and the high speed welding for the multielectrode welding process cannot be accomplished.
On the other hand, the increase of the number of the electrodes may rapidly increase the number of connections which are possible to choose theoretically, and this means difficulty of selection of the proper connections. Even if proper connections for the three-electrode welding process are applied to the four-electrode welding process, good results cannot be expected.